Where in the universe is the Milky Way?
Galaxies like ours huddle in clusters, and large-scale systems of galaxies, called superclusters, have vague boundaries that are difficult to define (especially from the inside): They’re all drawn to each other and interconnected in a web of filaments. Now for the first time, astronomers have constructed a map of the local universe. They’ve named our home supercluster Laniakea, Hawaiian for “immeasurable heaven.” The work was published in Nature this week.
By examining the motions of galaxies, a team of cosmic map makers led by R. Brent Tully from the University of Hawaii charted the distribution of matter in the universe to identify superclusters. A galaxy stuck between two superclusters will be caught in a gravitational tug-of-war. The balance of these forces determines the galaxy’s motion, and measuring the velocity helps define the region of space where each supercluster dominates. With a catalog of 8,000 galaxies' velocities, the team built a galactic distribution map and located the points where cosmic flows -- along which galaxies travel -- diverge.
The Laniakea supercluster, they found, is 520 million light-years in diameter and contains the mass of 100 million billion suns within 100,000 galaxies. Its name pays tribute to Polynesian navigators who used knowledge of the heavens to voyage across the Pacific Ocean.
In the image above, the colors represent density: red for high densities and blue for voids with little matter. Individual galaxies are shown as white dots, and velocity flow streams within the region gravitationally dominated by Laniakea are shown in white. Importantly, the orange line encloses the outer limits of these streams.
The team defined the edge of a supercluster as the boundary at which flow lines diverge: On one side of the line, galaxies flow towards one gravitational center, Nature explains, and beyond it, they flow towards another. “It’s like water dividing at a watershed, where it flows either to the left or right of a height of land,” Tully explains.
After mapping the boundaries of Laniakea, the team discovered that the Milky Way -- along with dozens of other galaxies in our Local Group -- resides at the outskirts of the supercluster. We’re the blue dot in the picture above.
“We have finally established the contours that define the supercluster of galaxies we can call home,” Tully says in a news release. “This is not unlike finding out for the first time that your hometown is actually part of much larger country that borders other nations.”
Laniakea also includes the Virgo cluster (our nearest neighbor, at 55 million light-years away) and Norma-Hydra-Centaurus. The latter is also known as the Great Attractor, which serves as a gravitation focal point that influences the motion of galaxies, Washington Post explains. Our supercluster is flowing towards the Shapley concentration of galaxies, in the upper left corner of the image above.
Image: SDvision interactive visualization software by DP at CEA/Saclay, France
Video: Nature Video